The present invention relates to resist materials for use in lithography, for example, in the production of integrated circuits and particularly to polymers having acetal or ketal functional groups containing silicon.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473; 4,115,128; and 4,173,470. These include alkali-soluble phenol-formaldehyde novolac resins together with light-sensitive materials, usually a substituted diazonaphthoquinone compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone compound acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the naphthoquinone compound undergoes a radiation induced structural transformation, and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
An alternative method for forming the pattern in a resist is referred to as chemical amplification. This method is described by C. G. Willson in Introduction to Microlithography (American Chemical Society, 1994, pp. 212-231). In this method, a photoacid generator is added to a polymer containing acid-labile groups. When a coating of this mixture is exposed to actinic radiation in an imagewise fashion, the photoacid generator in those areas struck by light will produce acid, and this acid causes a reaction of the acid-labile groups in the polymer. The polymer that has reacted in this manner is rendered soluble in aqueous base, and the image can be developed in the same manner as described above. Chemically amplified resist systems typically require a much lower dose of actinic radiation to effectively develop the pattern than do the novolac/diazoquinone type resist systems.
In most instances, the exposed and developed photoresist on the substrate will be subjected to treatment by a substrate-etchant solution or gas. The photoresist coating protects the coated areas of the substrate from the etchant, and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Resist resolution refers to the capacity of a resist system to reproduce the smallest features from the mask to the resist image on the substrate.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small features (on the order of two microns or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large-scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light-struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist composition, the exposed areas are the non-image areas. The light-struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence, positive resists are the choice for the manufacture of densely packed integrated circuits.
In the normal manner of using a positive photoresist, a single layer of this material is imaged to give a mask on the substrate, which can further be etched with a suitable etchant or used for deposition of materials, such as metals. However, due to the limitations of optical imaging systems, resolution of small patterns, on the order of 2 microns or less, is limited, particularly if topography is present on the substrate. It was discovered by B. J. Lin and T. H. P. Chang, J. Vac. Sci. Tech. 1979, 16, p. 1669, that this resolution can be further improved by using multilevel systems to form a portable conformable mask.
In a conventional two-layer resist system (B. J. Lin, Solid State Technol., 1983, 26 (5), p. 105), the substrate is first coated with a relatively thick planarizing layer to level any topography that might be present on the substrate. A relatively thin imaging layer resist is next coated on top of the planarizing layer. A latent image is deposited in the imaging layer by irradiation of this layer through a mask, and the desired pattern is formed in the imaging layer by subsequent development using conventional means. Pattern transfer to the substrate from the imaging layer through the underlying planarizing layer is finally accomplished by an anisotropic oxygen plasma etch (O2RIE). Hence much importance is given to the resistance of the imaging layer resist to O2RIE. Generally, those materials that form oxides upon O2RIE, for example those containing  greater than 10% by weight silicon, are considered to have high resistance to O2RIE.
The polymers of the present invention are characterized by having at least one pendent acetal or ketal functional group in which at least two substituents of the acetal/ketal carbon atom independently comprise at least one silicon atom. The compositions of the present invention are useful as positive working resist compositions, in particular as the top imaging layer in a bilayer resist scheme for use in the manufacture of integrated circuits. The incorporation of at least two silicon atoms in each monomeric unit enables the formation of a robust etch mask upon exposure to an oxygen plasma used in reactive ion etching processes.
In one aspect, the present invention provides a polymer comprising a polymeric backbone having at least one pendent acetal/ketal functional group having an acetal/ketal carbon atom in which at least two substituents attached to the acetal/ketal carbon atom independently comprise at least one silicon atom.
In another aspect, the present invention provides a resist material comprising compounds having the formula: 
wherein
R is a divalent connecting group or a covalent bond;
R1 is alkyl, aryl, aralkyl, or silyl;
R2 is a hydrogen atom, alkyl, aryl, aralkyl, or silyl;
R3 is alkyl, aryl, aralkyl, or silyl;
or any two of R1, R2, or R3 may be combined to form a cyclic group, with the proviso that at least two of R1, R2, or R3 comprise at least one silicon atom and wherein n is an integer greater than or equal to 1.
In another aspect, the present invention provides a method of forming resist patterns comprising the steps of:
a) providing a polymer comprising a polymeric backbone having at least one pendent acetal/ketal functional group having an acetal/ketal carbon atom in which at least two substituents attached to the acetal/ketal carbon atom independently comprise at least one silicon atom;
b) providing a substrate having an organic polymer base layer;
c) coating the silicon-containing polymer onto the organic polymer base layer of the substrate to form a top layer;
d) exposing the coated substrate to actinic radiation sufficient to form a latent image; and
e) developing the latent image.
In this application, xe2x80x9can acetal/ketal functional groupxe2x80x9d is represented by the formula: xe2x80x94Oxe2x80x94C(OR1)(R2)(R3); an xe2x80x9cacetal/ketal carbon atomxe2x80x9d is a carbon atom that is bound to both oxygen atoms in the acetal/ketal functional group; the acetal/ketal carbon atom is an acetal carbon atom if R2 is a hydrogen atom; if each of R2 and R3 is an alkyl or an aryl group, the carbon atom is a ketal carbon atom; and a xe2x80x9csubstituentxe2x80x9d is any group or covalent bond represented by R, R1, R2, and R3. It is to be understood that the terms xe2x80x9calkylxe2x80x9d, xe2x80x9carylxe2x80x9d, and xe2x80x9caralkyl groupsxe2x80x9d and the like include such groups that are substituted with other groups including but not limited to xe2x80x94Sixe2x80x94 or xe2x80x94Sixe2x80x94Oxe2x80x94 containing groups.